With a prevalence of information processing apparatuses such as a personal computer, a printing apparatus as am image generation terminal is also widely prevalent. In particular, an ink-jet printing apparatus which ejects ink from ejection openings onto the printing medium such as a paper to perform printing has various advantages, such as a non-impact and low noise printing system, a high density and a high speed printing operation, and easy application for color printing. From these points, the ink-jet printing apparatus is becoming a mainstream one in the field of a printing apparatus for personal use.
Such wide use of the ink-jet printing technology has required further improvement in print image quality. Particularly, since there is recently an environment where photos can be printed at home with ease by a print system, a quality of a print image not less degraded than in a silver halide photography has been demanded. In comparison with such sliver halide photography, a granular feeling in a print image is one of the conventional problems. On the other hand, various configurations of the print system for reducing the granular feeling are proposed.
For example, there is known an ink-jet printing apparatus using normal ink of cyan, magenta, yellow and black and further, ink of light cyan and light magenta which are lower in concentration of a color material such as dyestuff than the normal ink. Such an apparatus reduces a granular feeling by using ink of light cyan and light magenta in an area where the print density is low. In addition, in an area of high density, use of ink of cyan and magenta having a normal density realizes a wider color reproduction range and smooth gradation sequence properties.
Also, there is known a method of designing a size of a dot formed in the printing medium to be made small for reducing a granular feeling. For realizing this, there has been advancing the technology of reducing an amount of an ink droplet ejected from an ejection opening of the printing head. In this case, in addition to making an amount of an ink droplet be small, arranging many ejection openings in high density causes a high resolution image to be simultaneously obtained without impairing printing speeds.
Besides the aforementioned granular feeling reducing technology of focusing attention on the ink to be used, the technology of focusing attention on an area coverage modulation method is known as that of reducing a granular feeling by means of image processing. An ink-jet printing apparatus determines the number of dots to be formed to a pixel and carries out the printing according to the determined number of dots. In this processing, the multi-valued image data having density information is subjected to a quantization process to be finally converted into binary data, that is, data for determining execution/nonexecution of dot formation. The print image of an area having the extent which is macroscopically observed, the density or the gradation is expressed by the number and the arrangement of printed dots. Such expression of density or gradation is generally called as an area coverage modulation method. The area coverage modulation method includes various dot arrangements for expressing the same density. For example, there is known a dot arrangement according to an error diffusion method as described in a paper by R. Floid and L. Steinberg: “Adaptive Algorithm for Spatial Grey Scale”, SDI Int'l. Sym. Digest of Tech. Papers, paragraphs 36 to 37 (1975). In addition, as a method other than the error diffusion method, there is known a dot arrangement by an ordered dither method as disclosed in Japanese Patent No. 2622429 or Japanese Patent Laid-Open No. 2001-298617. These methods can create an image having a good visual perception in which an arrangement of formed dots is excellent in dispersion properties and low frequency components in a spatial frequency of the dot arrangement is few.
A so-called serial type of the ink-jet printing apparatus widely employs a multi pass method. It should be noted that words “pass” and “scan” used hereinafter have the same meaning. In the multi pass printing, an image data for a unit area is divided into data for each color and each pass and masks are widely used for the division.
FIG. 1 is a diagram for explaining the multi pass printing and schematically shows a printing head and dot patterns printed in a case of completing an image by four times of scans. In FIG. 1, P0001 denotes a printing head. Here, for simplifying its illustration and explanation, the printing head having sixteen ejection openings (hereinafter, also referred to as nozzle) is shown. The nozzle array is, as shown in FIG. 1, divided into four groups of a first to a fourth group, each including four nozzles for use. P0002 denotes a mask pattern where areas of a mask which permits printing (print permitting area) corresponding to each nozzle are painted in black. The mask patterns corresponding to four nozzle groups are complementary with each other. When the four mask patterns are overlapped, all the areas of 4×4 constitute the print permitting area. That is, four mask patterns is used to complete printing in all the areas of 4×4.
P0003 to P0006 denote arrangement patterns of formed dots and show the process in which an image is completed by executing plural times of printing scans. As shown in this pattern, in a multi pass printing, each printing scan forms dots based upon binary image data (dot data) generated with use of the mask patterns corresponding to nozzle groups respectively. In addition, each time the printing scan is completed, a printing medium is conveyed in an arrow direction by the width amount of one nozzle group. In this way, for areas corresponding to the width of respective nozzle groups in the printing medium, images of respective areas are completed by four times of printing scans.
According to the multi pass printing as described above, density unevenness due to a variation in an ejection direction or an amount of ink between plural nozzles possibly generated in the manufacturing process of a print head or to an error in paper conveying that is performed between printing scans can be hard to be observed.
It should be noted that in FIG. 1, the four-pass printing in which scanning the same image area (unit area) is executed four times is shown, but the multi pass printing is not limited to this four-pass printing. A two-pass printing in which an image is completed by twice of printing scans, a three-pass printing in which an image is completed by three times of printing scans, or a five or more-pass printing in which an image is completed by five or more times of printing scans may be applied.
In the multi pass printing, a number of printed dots in each printing scan can be adjusted or an operation frequency of a nozzle for which a trouble is easy to occur can be reduced, by changing an arrangement of a print permitting area in a mask pattern. That is, the multi pass printing can have modes in accordance with various purposes other than elimination of the above described density unevenness or bandings.
As described above, according to the recent ink-jet printing system, it is possible to output a stable image with a high quality at a high speed by wide variety of ink, implementation of various multi pass printings, adoption of a preferable area coverage modulation method (binarization method) and the like.
According to the studies by the inventor of the present invention, however, in the recent ink-jet printing system, with remarkable advancement of high speeding, high density and wide variety of ink kinds, it is confirmed that new problems which have no been confirmed so far are occurring. The high speeding, the high density and the increasing used ink kinds cause an increase in an amount of ink applied per unit time and unit area of the printing medium. In this case, depending on kinds of the printing medium, even if all the amount of ink applied can be finally absorbed, the absorbing speed may not correspond to an applying speed of the ink. More specifically, even if all the applied ink is finally absorbed and problems such as fixing properties or smears do not occur, ink droplets on the surface of the printing medium which are not yet absorbed may be contacted with each other during stages of scanning several times before completing an image. Then it is confirmed that this causes problems in a subsequent image.
For example, it is considered that a case where an image of blue expressed by cyan ink and magenta ink is printed by a multi pass printing system of a two-pass. In most of serial type ink-jet printing apparatuses, printing heads of fundamental four-color ink of cyan, magenta, yellow and black are arranged in parallel with one another in a primary scanning direction. In consequence, ink of each color is applied onto the same area of the printing medium by the same printing scan. More specifically, in the above case, ink based on data of cyan and magenta obtained by thinning the dot data of cyan and magenta respectively to be ½ is applied onto the printing medium with an extremely short time difference in the same printing scan. In this scanning, when the applied cyan and magenta ink droplets exist at the same position or at the neighboring positions, and the ink droplets are pulled with each other by mutual surface tensions so that a dot of two or more droplets of ink (hereinafter, also referred to as grain) may be formed. Once such grain is formed, the ink applied at a position close to the grain tends to be pulled to it. That is, the grain first generated becomes a core to gradually grow and finally forms a large grain. Such grain remarkably appears mostly in a high density area having a great applying amount of ink. Then, in a uniform image area, the state where that such large grains are irregularly dispersed is recognized, which causes a degradation of an image, so-called beading.
The phenomenon of the above grain is basically generated by applying a plurality of ink for a relatively short time in close proximity to each other and the degree of the pulling force depends on the mutual surface tensions of the ink. Formation of the grain, however, does not rely only on the mutual surface tensions of the ink. For example, in a case where the ink and a liquid that reacts with the ink and causes the ink to be coagulated are applied in the same scanning, the contacted, respective ink and liquid may be connected by a stronger chemical reaction to form a grain core.
In addition, in the case that the inks of the same color are applied during same scan such as inks of the same color is printed by using two arrays of the nozzles in the same scanning, the grain may be generated among the inks. Further, depending on an absorbing characteristic of ink into a printing medium, when inks to be applied at different scans in the multi pass printing is applied in close proximity to each other, the above grain may be generated.
One of causes of the aforementioned grain problem is an interference between a mask pattern for multi pass and an image data.
FIGS. 2A to 2D are diagrams explaining the problem caused due to interference. FIG. 2A shows a pattern of binary image data of cyan and FIG. 2B shows a mask pattern (print permitting areas are 50%) for a first pass among mask patterns of cyan for two pass printing. A size of the pattern of the binary image data in FIG. 2A is 4×4. On the other hand, the mask pattern in FIG. 2B is a mask arranging print permitting areas in size of 4×4 and corresponds to the pattern of the binary image data one to one.
In this case, at the first pass, a dot pattern shown in FIG. 2C which is an AND data of the mask pattern and the binary image data pattern is to be printed. More specifically, the binary image data shown in FIG. 2A shows four dots to be formed, but in fact, the number of dots formed at the first pass is zero. In contrast, at the second pass shown in FIG. 2D, all of the remaining four dots are formed. In this way, interference between the mask pattern and the binary image data (dot data) occurs, thereby possibly bringing about various problems, such as the problem that an original effect of the multi-pass printing is not achieved sufficiently. In addition to the example shown in FIGS. 2A to 2D, there may occur the reverse case, that is, a case where at the first pass, four dots are formed and at the second pass, the number of the formed dot is zero. In addition, this interference possibly occurs in combinations of various binary image data patterns and corresponding pass mask patterns regardless of a size of the data.
The interference as described above possibly occurs in parts of the mask processing for each scan to the entire binary image data. Then, a deviation of the dots to a certain scan due to the interference shown above may lead to occurrence of a grain at the time of generating an image (hereinafter, also referred to as intermediate image) at a half way to completing the image by plural times of scans in the aforementioned multi pass printing.
According to the conventional system as described above, in a case of completing an image by superimposing plural divided images having a different formation timing, since dispersion of dot arrangement obtained by superimposing images among the divided images for respective formation timings is not considered, occurrence of the grain in the intermediate image can not be restricted.